The invention relates to an illumination system for illuminating an object with EUV radiation, which system comprises an EUV radiation source unit and at least one EUV radiation-reflecting mirror having a multilayer structure of first layers of a first material alternating with second layers of a second material.
The invention also relates to a lithographic apparatus provided with such an illumination system, and to a method for manufacturing devices.
A lithographic apparatus is used, inter alia, in the manufacture of integrated electronic circuits, or ICs, for imaging an IC mask pattern, present in a mask, each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic apparatus may also be used in the manufacture of other devices like, for example, liquid crystalline display panels, integrated or planar optical systems, charge-coupled detectors (CCDs) or magnetic heads.
Since it is desirable to accommodate an increasing number of electronic components in an IC, increasingly smaller details, or line widths, of IC patterns must be imaged. Thus, increasingly stricter requirements are imposed on the imaging quality and the resolving power of the projection system which is usually a lens system in the current lithographic apparatuses. The resolution, which is a measure of the smallest detail which can still be imaged satisfactorily, is proportional to xcex/NA, in which xcex is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolution, the numerical aperture may, in principle, be increased and/or the wavelength may be reduced. In practice, an increase of the numerical aperture, which is currently already fairly large, is not very well possible because this reduces the depth of focus of the projection lens system, which is proportional to xcex/NA2, while it becomes too difficult to correct for the required image field.
The requirements to be imposed on the projection lens system may be alleviated, or the resolution may be increased, while maintaining these requirements, if a step-and-scanning lithographic apparatus is used instead of a stepping lithographic apparatus. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one operation and imaged as a whole on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate are provided with the mask pattern. In a step-and-scanning apparatus, only a rectangular or circular segment-shaped area of the mask pattern and hence also a corresponding sub-area of a substrate IC area is each time illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, while taking the magnification of the projection lens system into account. A subsequent area of the mask pattern is then each time imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of the next IC area is moved into the projection beam and the mask is set to its initial position whereafter said next IC area is scan-illuminated via the mask pattern.
If even smaller details are to be imaged satisfactorily with a stepping or a step-and-scanning lithographic apparatus, it is still possible to reduce the wavelength of the projection beam. In the current stepping and step-and-scanning apparatuses, deep UV (DUV) radiation, i.e. radiation having a wavelength of the order of several hundred nanometers, for example 245 nm or 193 nm from, for example, an excimer laser is already used. Another possibility is the use of extreme UV (EUV) radiation, also referred to as soft X-ray radiation, with a wavelength in the range of several nm to several tens of nm. Extremely small details, of the order of 0.1 xcexcm or less, can be satisfactorily imaged with such a radiation.
Since there is no suitable material with which lenses can be made available for EUV radiation, a mirror projection system must be used for imaging the mask pattern on the substrate, instead of a hitherto conventional projection lens system. For forming a suitable illumination beam from the radiation of the radiation source unit, mirrors are also used in the illumination system.
The article xe2x80x9cFront-end design issues in soft X-ray projection lithographyxe2x80x9d in Applied Optics, vol. 32, no. 34, Jan. 12, 1993, pp. 7050-56 describes a lithographic projection apparatus in which EUV radiation is used. The illumination system of this apparatus comprises three mirrors and the imaging, or projection, system comprises four mirrors. The radiation source unit comprises a high-power laser generating a plasma in a medium emitting EUV radiation. This radiation source unit is known as Laser Produced Plasma Source (LPPS). Said medium may be a solid, a liquid or a gaseous medium, and the generated EUV radiation has a wavelength of 13 nm.
It is a great problem in EUV lithographic apparatuses to illuminate the substrate with a sufficiently high intensity. A first cause of this problem is that radiation sources emitting radiation at the envisaged wavelength, in the range of 13 nm, are not very efficient and only supply a limited quantity of radiation. Moreover, the mirrors are considerably less than 100% reflecting. Each of these mirrors has a multilayer structure whose composition is adapted as satisfactorily as possible to the wavelength of the projection beam used. Examples of such multilayer structures are described in U.S. Pat. No. 5,153,898. A multilayer structure which is often referred to in literature is the structure consisting of silicon layers alternating with molybdenum layers. For radiation supplied by a plasma source, these layers theoretically have a reflection of the order of 73% to 75%, but in practice, the reflection is currently not larger than 65%. When said number of seven mirrors is used with a reflection of 68% each, only 6.7% of the radiation emitted by the source reaches the substrate. In practice, this means for a lithographic apparatus that the illumination time must be relatively long so as to obtain the desired quantity of radiation energy on an IC area of the substrate, while for a scanning apparatus the scanning rate is relatively short. However, it is essential for these apparatuses that the scanning rate is as high as possible and the illumination time is as short as possible so that the throughput, i.e. the number of substrates which can be illuminated per unit of time, is as high as possible.
It is an object of the present invention to provide an illumination system in accordance with a novel concept, which illumination system is particularly, but not exclusively suitable for use in a lithographic apparatus. This illumination system is characterized in that the radiation source unit is provided with a medium bombarded by an electron beam, which medium comprises at least a material which is equal to one of the materials of said mirror.
As regards the wavelength, the invention provides an ideal combination of radiation source and mirror. Since the medium bombarded by the electron beam comprises the same material as one of said first and second layers of the mirror, the radiation source unit emits EUV radiation at the wavelength for which the mirror is maximally reflecting. For this wavelength, the reflection of the mirror is now 78% instead of said theoretical 73% to 75%, hence 3% to 5% larger. For a lithographic apparatus with seven mirrors, this means that 17.6% of the radiation emitted by the source can reach the substrate instead of 11% in the case where the reflection of the mirrors is 73%, which is a gain factor of 1.6. The radiation source unit of the novel illumination system supplies a radiation intensity which is comparable with that of an LPPS, but can be given a more compact design. Moreover, this radiation source unit is cleaner, i.e. the medium does not release particles which may be deposited on the mirrors and contaminate them.
Different radiation sources may be used within the concept of the invention. A first embodiment of the illumination system is characterized in that the medium of the radiation source unit consists of a single material which emits Cherenkov EUV radiation upon electron beam bombardment.
As described in the article by V. A. Bazylev et al. xe2x80x9cX-ray Cherenkov radiation. Theory and experimentxe2x80x9d in Sov. Phys. Jetp 54 (1981) page 884, Cherenkov radiation is produced if a material is bombarded with electrons whose velocity is larger than the phase velocity of the Cherenkov radiation in the medium. This article comprises a theoretical discourse about the Cherenkov radiation and states the conditions under which this radiation may be produced. Generally, a large change of the dielectric constant occurs for a material around the molecular absorption edges of the material. If this material is bombarded with a high energetic electron beam, the intensity of the Cherenkov radiation has a maximum at that energy at which the dielectric constant has a minimum. The selection of the absorption edge is determined by the desired wavelength of the Cherenkov radiation. The Bazylev article states carbon as an example of a solid-state medium in which Cherenkov radiation can be generated. Although the use of the radiation in a lithographic apparatus is mentioned, the use of silicon as a medium for generating the radiation is not mentioned. Moreover, the energy of the electrons is of the order of 1 GeV in the Bazylev article, and the generated Cherenkov radiation is hard X-ray radiation, not soft X-ray radiation or EUV radiation.
A second embodiment of the illumination system is characterized in that the medium of the radiation source unit comprises a multilayer structure of said first and second materials which emits EUV transition radiation upon electron beam bombardment.
As described in the article by A. E. Kaplan et al. xe2x80x9cX-ray narrow-line transition radiation source based on low-energy electron beams traversing a multilayer nanostructurexe2x80x9d in Phys. Rev. E 52 (1995) page 6795, transition radiation is produced if an electron beam whose electrons have an energy of the order of 10 MeV is passed through the transition between two materials having different dielectric constants. Also when generating transition radiation, use is made of the large change of the dielectric constant occurring around a molecular absorption edge. The radiation source unit described in the Kaplan article supplies hard X-ray radiation and is intended as a replacement for a synchrotron. The Kaplan article is limited to a theoretical discourse about the radiation source unit and does not describe a mirror system for further guiding the generated radiation and forming a suitable radiation beam.
A preferred embodiment of the illumination system using a Cherenkov radiation source is characterized in that the medium consists of silicon, and in that the first and the second material of the mirrors are silicon and molybdenum, respectively.
When using silicon as a medium, radiation having an energy of 99.7 eV and a wavelength of 12.44 nm is produced. For this wavelength, a mirror with silicon and molybdenum layers has a maximal reflection. Cherenkov radiation of the same wavelength is obtained by bombarding the Si medium with electrons having an energy of several MeV.
A preferred embodiment of the illumination system using a transition radiation source is characterized in that the materials of the medium are silicon and molybdenum.
The transition radiation having a wavelength of 12.44 nm is obtained by bombarding the Si/Mo transition(s) with electrons having an energy of several tens of MeV.
The invention also relates to a lithographic projection apparatus comprising an illumination system, a mask holder for accommodating a mask, a substrate holder for accommodating a substrate, and a mirror projection system for imaging a mask pattern, present in the mask, on the substrate. This apparatus is characterized in that the illumination system is a system as described hereinbefore.
This projection apparatus is further preferably characterized in that the mirrors of the mirror projection system have the same multilayer structure as the mirrors of the illumination system.
Optimal use is then made of the inventive idea.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.